1. Field of the Invention
This invention relates to electrical signal isolators for translating a signal level without establishing any conductive connection between input and output. More particularly, this invention relates to half-wave transformer-coupled isolators.
2. Description of the Prior Art
Electrical signal isolators are used in a wide variety of electrical systems, to enable signal levels to be translated from one part of a system to another without permitting the flow of direct current therebetween. Isolators are used in diverse applications such as industrial process control systems, making medical measurements of the human body, and so on.
A number of different types of isolators have been used in the past. Many isolators employ transformer coupling to establish a signal path without DC conductivity. Reference in that regard may be made to U.S. Pat. Nos. 3,946,324 (L. R. Smith), 4,054,829 (T. J. Searle), and 4,066,974 (C. J. Reinhard) simply as illustrative of different kinds of isolators; many other kinds of transformer-coupled isolators are described in still other patents and publications.
One type of transformer-coupled isolator which has significant advantages is that referred to as a half-wave isolator. In such an arrangement, the d-c input signal is modulated (chopped) by an electronic switch connected in series with the transformer primary. The switch typically is driven by an oscillator carrier signal so as to close the series circuit to the transformer primary on alternate half-cycles. A corresponding demodulator switch is employed in series with the transformer secondary winding to recover the d-c signal level. The demodulator switch is driven by the same oscillator signal as the modulator switch, so as to establish synchronism between the two switches.
Such half-wave signal isolators have been used extensively heretofore. However, prior isolators of that type have suffered from certain serious problems which have made such devices less than satisfactory, particularly for applications with demanding performance requirements. For example, such devices commonly are subject to undesirable drift, especially as a result of variations in ambient temperature. Adverse effects can be caused by a variety of factors, such as drift of the oscillator frequency, drift of the reactance of the coupling transformer, temperature dependence of the switch resistances, and so on. The present invention is directed at minimizing such adverse effects.